This invention relates to polymer compositions comprising transparent polymer blends, methods for their preparation, and articles made therefrom.
Polycarbonates and polyesters, especially poly(alkylene dicarboxylates) (e.g. poly(ethylene terephtahlate) and poly(butylene terephtahlate), and blends thereof are widely employed classes of materials, in part because of their excellent physical properties. However, long term color stability among polycarbonates, polyesters, poly(alkylene dicarboxylates), and blends thereof remains a persistent problem. Color instability is evidenced by yellowing, and loss of transparency and attractiveness of molded articles comprising such polymers. Loss of gloss can also be an undesirable long term phenomenon. In addition, if selected for use in certain applications such as automotive body panels, polycarbonates, polyesters, poly(alkylene dicarboxylates), and blends thereof must display toughness in impact tests while retaining heat resistance up to a minimum of about 80° C. and adequate injection moldability.
Yellowing of polycarbonates and polyesters is caused largely by the action of ultraviolet radiation, which is why such yellowing is frequently designated “photoyellowing”. Numerous means for suppressing photoyellowing in polycarbonate-containing materials have been employed. The most common method for suppressing photoyellowing is by adding one or more ultraviolet absorbing compounds (UVA's). For the most part, UVA's are low molecular weight compounds, which must be employed at relatively low levels, typically below 1% by weight, to avoid degradation of such physical properties of the polymer as impact strength and heat distortion temperature. Another method for suppressing photoyellowing comprises blending other polymers with polycarbonates and/or polyesters to improve their resistance to degradation by ultraviolet radiation and to retard loss of gloss. The resistance of a material to photodegradation and loss of gloss is collectively designated “weatherability”. Examples of “weatherable” blends include blends of polycarbonates with copolyesters, wherein the copolyesters comprise “resorcinol iso/terephthalate” structural units (See structure I below), optionally in combination with “soft block” ester units derived, for example, from an aliphatic or alicyclic dihydroxy compound or dicarboxylic acid as described in U.S. Pat. No. 6,143,839. Unfortunately, such weatherable blends are typically immiscible. Therefore their use is often limited to situations in which transparency is not required. Another concern common to organic polymers is their susceptibility to attack by organic liquids, a limitation which affects the uses to which such polymers may be put.
Because of the above-mentioned problems and limitations, it is of interest to produce a wider variety of weatherable and weatherability-improving compositions. It remains of interest, therefore, to develop new polymer compositions that can be employed to make molded articles or films which display both resistance to photodegradation as well as enhanced chemical resistance. Specifically, it would be desirable to find compositions having polycarbonate-like toughness and moldability, while simultaneously possessing good resistance to degradation by certain chemicals, and excellent weatherability. It would further be desirable if such compositions were to retain their desirable transparency and surface gloss characteristics over an extended time period.